Lenovo ThinkServer RD240 MegaRAID SAS Software User Guide - Page 132

◊ Read Only: Allow read-only access. This is the default. ◊ Blocked: Do not allow access. • Read Policy: Specify the read policy for this virtual drive: ◊ Normal: This disables the read ahead capability. This is the default. ◊ Ahead: This enables read ahead capability, which allows the controller to read sequentially ahead of requested data and to store the additional data in cache memory, anticipating that the data will be needed soon. This speeds up reads for sequential data, but there is little improvement when accessing random data. ◊ Adaptive: When Adaptive read ahead is selected, the controller begins using read ahead if the two most recent drive accesses occurred in sequential sectors. If the read requests are random, the controller reverts to Normal (no read ahead). • Write Policy: Specify the write policy for this virtual drive: ◊ WBack: In Writeback mode the controller sends a data transfer completion signal to the host when the controller cache has received all of the data in a transaction. This setting is recommended in Standard mode. ◊ WThru: In Writethrough mode the controller sends a data transfer completion signal to the host when the drive subsystem has received all of the data in a transaction. This is the default. ◊ Bad BBU: Select this mode if you want the controller to use Writeback mode but the controller has no BBU or the BBU is bad. If you do not choose this option, the controller firmware automatically switches to Writethrough mode if it detects a bad or missing BBU. Caution: Lenovo allows Writeback mode to be used with or without a battery. It is recommended that you use either a battery to protect the controller cache, or an uninterruptible power supply (UPS) to protect the entire system. If you do not use a battery or a UPS, and there is a power failure, you risk losing the data in the controller cache. 4-56 WebBIOS Configuration Utility